The present invention relates to the recovery of heat from the flue gas of a combustion furnace, and more particularly to the utilization of the heat recovered from the flue gas to increase the steam production from a steam boiler or to increase the power production capability of a power generation plant.
In the typical combustion system of a steam boiler or power generation plant in which burning fuel discharges smoke, flue gas and other contaminants into the atmosphere, a considerable amount of heat is also discharged and therefore wasted. It is therefore highly desirable to be able to recover and utilize the heat that is being wasted to the atmosphere. Many fuels which are currently of interest contain a large amount of water which must be evaporated from the fuel. Such fuels may also contain considerable hydrogen, which when burned produces water vapor. Some examples of such fuels are garbage, natural gas, wood pulp, waste liquor from a paper plant, and others. All of these fuels burn and produce a considerable amount of water vapor in the flue gas. Since water vapor has a very high latent heat of evaporation, it would be very desirable to collect this heat and use it in the steam generation cycle or power generation cycle. In particular, the heat of condensation of the flue gas may be recovered by reducing the flue gas temperature to a value as low as 80.degree. F. When the flue gas temperature is lowered to 80.degree. F., very little water vapor remains in the gas, and a great amount of heat is removed from the gas, which can be used at various points in the thermodynamic cycle. By way of illustration, flue gas leaving the boiler or furance at a typical temperature of 600.degree. F. and having a dew point of gas with water evaporated in the gas as high as 150.degree. F. has an enthalpy or heat content of 275 Btus per pound of dry gas. If the gas is cooled to a temperature of 80.degree. F., the enthalpy is reduced to approximately 43.7 Btus per pound. Thus, approximately 232 Btus per pound of gas can be collected if the gas temperature is reduced from 150.degree. F. to 80.degree. F.
It is known in the prior art to pass the feed water entering a steam boiler into thermal contact with the flue gases exiting the combustion furnace of the boiler to economize the heat in the flue gases. Furthermore, it is known to preheat the water entering a heat recovery unit so as to maintain the temperature in the heat recovery unit above the condensation temperature of the gas prevent corrosion by the gas of the heat recovery tubes. Examples of such prior art are U.S. Pat. Nos. 1,219,320 to Jacobus; 2,699,759 to Kuhner; 3,675,423 to Vidal et al; 4,173,949 to Roethe; and 4,370,949 to Beckett. It is also known to cool gases exiting a heat exchanger by contacting the flue gases directly with water in a gas washing or scrubbing apparatus. Examples of a direct contact scrubbers are found in U.S. Pat. Nos. 3,884,162 to Shuster; 3,848,548 to Bolejack, Jr. et al.; 3,457,883 to Ankersen; 3,482,533 to Ankersen; 3,665,871 to Schwartz, Jr. et al; and 3,812,793 to Solomon. In the flue gas heat recovery systems of the prior art, however, a great deal of the heat from the flue gas is wasted. Much of the wasted heat is retained in the water vapor in the flue gas, which has not been cooled to a sufficiently low temperature to extract all the heat which it contains. Furthermore, the heat which is recovered is not applied to the point in the thermodynamic cycle where it will be most efficiently recovered into increased steam production for a steam boiler or increased power production capability for a power plant.